feat: complete remaining jetstream parity implementation plan

tests/NATS.Server.Tests/JetStreamExpectedHeaderTests.cs

@@ -0,0 +1,14 @@
+namespace NATS.Server.Tests;
+
+public class JetStreamExpectedHeaderTests
+{
+    [Fact]
+    public async Task Expected_last_sequence_mismatch_returns_error()
+    {
+        await using var fx = await JetStreamApiFixture.StartWithStreamAsync("ORDERS", "orders.*");
+        _ = await fx.PublishAndGetAckAsync("orders.created", "1");
+
+        var ack = await fx.PublishWithExpectedLastSeqAsync("orders.created", "2", expectedLastSeq: 999);
+        ack.ErrorCode.ShouldBe(10071);
+    }
+}